Thermally adaptive wall to enhance indoor comfort and energy performance

In the present work, a new environment-adaptive wall is proposed, based on an inside and outside radiant panel with pipes drowned on the panels themselves and hosting a heat-carrying liquid pushed by a pump. The purpose consists of transferring heat across the thickness of the wall, in the direction required for energy saving and enhancement of inner indoor thermal comfort. A Computational Fluid Dynamic (CFD) analysis is reported and the results are applied on a single zone box-shaped building, where a whole-year study is implemented by means of the transient simulation tool TRNSYS. The efficiency of this solution respect to the state-of the-art static walls is finally discussed. A concept of a laboratory setup of an adoptive wall along with first result are presented and discussed.


Introduction
Active systems designed to increase the energy performance of the building envelope represent one of the research frontiers in the construction sector [1].Thermally adaptive walls for building applications have been divided into the following types: PCM integrated walls, ventilated walls, dynamic insulation materials integrated walls, switchable insulations systems integrated walls, PCM integrated dynamic insulation walls [2].Within this framework, the focus on the cooling requirements is relatively more recent respect to the massive investigations developed along the last decades on materials insulation properties [3,4].
Transparent surfaces knew a flourishing of solutions directed towards an active behaviour respect to the environmental conditions, such as photochromic, thermochromic or electrochromic glasses [5], aimed at defining a system that changes ideally its optic-energy properties as required by the users.
On the other hand, apart from phase change materials, a gap seems existing for the opaque fraction of the envelope respect to the glazing, as only a few components have been proposed in this direction such as, for instance, walls with modifiable air gaps, which effect on the overall varying thermal insulation properties is limited and difficult to control [6].
A system for adjusting the temperature of a building is described in [7], where two series of pipes are positioned at the two faces of a wall, i.e., straddling the insulating wall.Considering the summertime behavior of the walls, the circulation of the fluid is promoted at night by bringing heat from the inner portion of the building to the outside.
Another solution for internal walls was presented in [8], where concrete wall with embedded PEX pipes inside (Thermal Energy Storage System) was combined with copper pipes and rigid foam on both boundaries (Active Insulation System).Concrete with pipes inside cooperating with a chiller works as a thermal storage unit.When the stored energy is needed, it is transferred by the water flow though the copper pipes of the active insulation on both sides of the wall to reduce HVAC loads.
In the work of Bailey [9] a heat exchanger panel comprising an insulating layer mounted adjacently to a solid wall of a building is described.The setup comprises ducts connected by means of a fluid circuit to solar panels located on the roof.The heat exchanger panel can be part of an air conditioning system for a building consisting of a combination of a radiant heating or cooling system to best keep the desired temperature in the building.The system can include solar panels and radiant cooling plates.
The solution described in the present work consists of two radiant panels embedded in the outer and inner surface of any insulated wall [10].The functioning concept is based on transferring the heat from and to the inner environment, according to energy and comfort requirements, controlling the pump that moves (or not) the fluid inside the circuit.The next paragraph describes in more the detail the working strategy.

Environment adaptive wall: the rationale
The environment adaptive wall core is obtained through the immersion in any horizontal, vertical or oblique wall made of any material, of pipes made of plastic, metal material, or other materials usually used for transporting liquids.A heat transfer fluid flows, moved by a thrust member such as a pump to transport the heat in the direction of the thickness of the wall, from the inside to the outside, and vice versa.
The control of the system is associated with two temperature sensors (Te and Ti), one located in the outdoor environment, and one located indoors: their differential value guides the on and off switching of the pump (fig.1).In wintertime, if the walls are properly insulated, the system does not work.The solar radiation in the late fall and early days of spring can heat the outer surfaces of the building envelope and this heat can be immediately transported into the buildings, by means of the fluid flow.During the hot season, the outdoor temperature is lower than the indoor thermal wellbeing setpoint for a non-negligible number of hours in the day (especially the night-time hours).The transmission of heat (from the inside to the outside) in such periods is facilitated by the proposed device, thanks to the significant reduction of the wall thermal insulation and thermal inertia.
The adaptive wall further includes a control unit which acquires the data measured by the inner and outer temperature probes, compares them, and controls the pump.The central layers of the wall could be made of any material, while the two border layers where the pipes are embedded are realised with high thermal conductivity solutions, such as, for instance, plasterboard, cement, and other construction materials with similar characteristics (fig.2).

Figure 2. Application of the adaptive wall to various envelopes.
The indoor and outdoor temperature probes should be positioned in the middle of the wall, away from heat bridges and away from heat sources, embedded in the inner outer layers respectively.
The adaptive wall could be divided into several parts, each crossed by an independent trajectory of duct, and there could be an independent control of each circuit (fig.3).Preferably, the pipes forming the duct are made of a material selected from plastic, copper or iron, but it has not particular requirements respect to the common materials used in radiant panels.The heat transfer fluid is selected from pure water and water with the addition of other substances, such as glycol, which inhibits the freezing.The additive has varying percentages between 10% and 40% according to the climate zone where the device is installed.
The layout of the pipes could be designed in different ways.Figures 4b, 4b and 4c show three examples of pipe paths: 1) Sparse roundtrip pipes: the duct moves forward and backward in one face before going to the opposite side; 2) Sparse serpentine pipes: the duct moves only in one direction in one face before going to the opposite side; 3) Dense roundtrip pipes: the duct moves forward and backward in one face before going to the opposite side, the step of the path is lower respect to the first configuration.

CFD simulations of various configuration
The proposed adaptive wall has been simulated with a finite volumes code [11], following what will be tested in an experimental setup.
The model is a multi-layer highly insulated vertical wall, sized 1.23 m x 1.48 m, with a total thickness of 0.17 m.Starting from the inner environment, the first layer is made of gypsum and it hosts the first pipes layout.Then, an insulating layer of wood wool (with high thermal inertia) and another one of polystyrene are inserted, to finish with gypsum again (hosting the second path of the pipes) and a protective final layer of plaster on the outside.The thermophysical properties of each material are retrieved from the Standard ISO 10456 [12].
The inner diameter of the pipes is 8 millimetres, as this results one of the commercially available size for ducts used in radiant floor or walls.
The meshing process needed a strong effort to find a compromise between number of cells and their quality, because of the different granularities among the extension of the domain: the wall layers and the pipes.Figure 5 shows the grid for the dense roundtrip configuration.
The domain is meshed with tetrahedral cells; the total number of nodes are 2,223,396, with 10,583,055 elements.The mesh geometric characteristics has been checked with the element quality indicator, which reached its minimum value at 0.021; thus, it can be concluded that the wall is satisfactorily transformed in the computational domain.A mesh independence control was executed with a coarser grid, obtaining a heat flow rate that differed less than 1% respect to the chosen number of cells.All the domain is solid, except for the liquid inside the pipe: a mixture of water (80%) and glycol (20%) is considered.The boundary conditions in the border faces are taken from the Standard ISO 6946 [13], e.g.7.69 W/m 2 K in the inner surface and 25.00 W/m 2 K in the outer one.The other lateral surfaces are considered adiabatic.
The fluid speed is defined by setting the pressure drop guaranteed by the pump that pushes liquid, therefore, the flow pattern depends on the pressure drop itself.
The steady-state simulations demonstrated that the wall global thermal transmittance may be raised by one order of magnitude when the fluid is in motion respect to the case with the fluid still [14]; furthermore, from the stationary analysis it emerged that a good compromise between the system, efficiency and the energy needed for the pumping is obtained with a fluid speed of 0.20 m/s.A simulation with a fluid speed of 0.40 m/s has been also conducted, showing negligible differences in terms of heat transfer respect to the case of 0.20 m/s.Since instead, the pressure drops increases of a factor close to 3, the speed of 0.20 m/s has been chosen.
Starting from these results, transient simulations have been implemented.The hot season case is following described, but the procedure and the results may be extended also to other periods of the year.
The inner temperature is fixed at a summer setpoint (26°C), while the outer temperature is taken from the weather data for the city of Perugia (Italy), taking also into account of the solar radiation for a southfacing wall with an absorption coefficient of 0.60 (sol-air temperature).Hypothesizing that the trend of the sol-air temperature is repeated for three days, the condition of stabilised periodic regime is fulfilled.Therefore, the transient simulation is repeated for three days with the same boundary conditions, but only the last 24 hours are analysed, when the initial transitory phase has disappeared.
In fig.6 the thermal field of the analysed wall is reported during the first 8 hours of the days under analysis.
It is clear that the wall guarantees a free colling effect in the opaque wall as long as the outer temperature is lower than the inner setpoint temperature.When the boundaries changes (at 08:00), the pump stops and the wall turns back to be an insulating envelope.
The following graph (fig.7) reports the results of two control strategies: one with the pump always on and the other with the pump that switches on only if the outer temperature is lower than the inner one.The heat flow entering the building is also reported, compared with the wall without the pipes.
In the configuration without the pipes, the wall cools the inner environment with an average value of 0.9 W/m 2; the cooling effect is enhanced to 5.8 W/m 2 with the pump always on, to arrive at 6.6 W/m 2 when the pump controlled by the boundary conditions.No significant variations have been found varying the speed of the liquid.

Description of the laboratory setup for the CFD validation
The performance of the proposed system will be also validated, beyond the simulations, through a hot box system.This device is made of a calorimetric chamber consisting of two rooms separated with a high thermal resistance wall, called surround panel, into which a specimen is mounted.The measurement setup is given in fig.8.The two rooms simulate internal and external air conditions.The chamber is equipped with the measurement and control system which allows to set, control and measure the following data: air temperatures on both sides of the specimen, mean air velocities as well as surface temperatures on the demanded faces of the specimen.Within the chamber, it is possible to set steady-state normative conditions to perform measurements of thermal transmittance of the specimen.On the other hand, the conditions can be set as transient, simulating dynamic conditions, for example hourly external or internal air changes.The external air temperatures can be set as sol-air temperatures.The temperature sensors are T-thermocouples with the accuracy of 0.1 K.All the equipment is connected to the Ahlborn Almemo control and acquisition program with PID controller.For the specific measurements, a sandwich wall with a coil embedded in the outer layers of the partition will be prepared.The size of the sample is 1230 mm x 1490 mm.The arrangement of the coil is shown in figg.9a and 9b, while the cross-section of the test sample is shown in fig.9c.The dimensions of the sample are determined by the size of the hole for mounting the measuring samples in the calorimetric chamber.The circulation pump with flow meter will be mounted in the lower part of the tested wall on the warm side.A pump with a motor isolated from the pumped liquid will be used to avoid the thermal effect of heat from the engine on the pumped liquid.

Conclusions and future work
The proposed solution provides the possibility of obtaining variable insulation of the building insulated external wall in the range of one order of magnitude.This gives the possibility of adjusting the heat flux flowing through the wall to the thermal conditions prevailing at a given moment, which allows both heat transfer from the external environment to the building and in warm periods to give off heat from the building to the external environment.The change in the thermal properties of the partition can be very quickly and easily achieved by controlling the operation of the pump forcing the flow of fluid in coils embedded in the walls.Future research will focus on measurements at a laboratory set up, which will give the opportunity to practically assess the accuracy of the developed CFD model and will allow to establish guidelines for the practical implementation of such systems in construction.

Figure 1 .
Figure 1.The proposed adaptive wall principle.

Figure 3 .
Figure 3.The proposed adaptive wall principle.

Figure 4 .
Figure 4. A) Sparse roundtrip configuration of the adaptive wall.B) Sparse-serpentine configuration of the adaptive wall.C) Dense roundtrip configuration of the adaptive wall.

Figure 5 .
Figure 5. Grid of the computational domain for the dense roundtrip configuration.

Figure 6 .
Figure 6.Trend of the thermal field created by the system during night-time in a summer typical day.

Figure 7 .
Figure 7. Sol-air temperature and specific heat fluxes entering the indoor environment for the different walls for a summertime day.

Figure 9 .
Figure 9. A) Pipe layout.B) Front view of thermocouples placement.C) Cross section with thermocouples placement.